This invention relates to a device for controlling the speed of movement of a mobile member.
In particular, this invention relates to a device arranged to control the speed of movement of a mobile carriage of a plastics moulding machine, of a machine tool or of a hydraulic press etc., relative to a fixed machine support member.
In the machine tool field in general, and in moulding machines in particular it is often necessary both to know the position of a particular mobile carriage at any moment in a processing cycle, and to control the carriage speed.
If the carriage is driven by a hydraulic system, such machines are provided with a pressurised fluid source which feeds the fluid through a conduit to a carriage drive member. At present, the speed of this latter is controlled indirectly by controlling the flow rate of the fluid fed to the drive member by means of flow regulators mounted in series along said conduit.
A first type of regulator device is known comprising constrictions through which the pressurised fluid passes. The opening in these constrictions is adjustable in such a manner as to obtain a fluid flow rate which gives a corresponding required carriage movement speed. However, such constrictions give rise to a pressure drop along the conduit, such that it is necessary to have a source pressure upstream of the regulator which is considerably higher than that required on average by the drive member. As the fluid quantity not fed to the drive member is bled off through a valve at this pressure, there is consequently a considerable wastage in power associated with the fluid bled off, and there is also a substantial pressure drop along the conduit at said constrictions. Furthermore, if a resisting force exceeding a limiting value opposes the carriage during a processing cycle, the pressure downstream of the regulator can be insufficient, and therefore give rise to undesirable slowing down of the carriage.
A second known type of flow regulator, designed to obviate the drawbacks of the said first type, comprises substantially a flow transducer mounted in said conduit, to provide at its output an electrical signal proportional to the flow rate in the conduit, and further comprising a valve disposed upstream of the flow transducer and arranged to bleed off a variable quantity of the pressurised fluid supplied by the feed source. The electrical signal supplied by the transducer is compared with an electrical reference signal, and the resultant electrical signal is used to control the bleed valve in such a manner as to maintain the fluid flow rate constant, and thus the carriage speed constant as the resistant force opposing the carriage varies. However, this second type of regulator device has a drawback due to its poor response time. In this respect, as the flow transducer is generally mechanical, it has an inherent inertia which introduces excessively high response times. Consequently, the pressure adjustment resulting from a sudden resistant force opposing the carriage is slow, and there is a consequent fall in the instantaneous speed. Moreover, as the available regulators always have mechanical moving parts, these latter wear with time, and this gives rise to fluid leakages, because of which after an initial period of activity, continuous maintenance is required.
Finally, the flow regulators available commercially are of rather high cost, which when associated with the cost of the frequent maintenance makes the use of such regulators particularly expensive.